1. Technical Field
The present invention relates to the technical field of filled polymer castings and moldings and particularly to simulated marble products, such as plumbing fixtures, docorative panels, and the like, made from filled polymers.
2. Prior Art
It has long been recognized that marble products may be substituted by cast polymer castings and moldings. Such products offer substantial savings in cost, weight, labor requirements, and a variety of other features. Enhanced physical properties, such as resistance to cracking, impact, and the like have also been among the objectives of such efforts to provide substitutes for marble products. Considerable efforts have been made to achieve products which simulate the appearance of marble, and particularly polished marble through direct casting of desired product shapes. Such products have met with considerable success and market acceptance.
In general terms, such products are based on a polymer binder, usually a thermoset polymer binder, and a high loading of particulate or granular fillers, particularly calcium carbonate. Such formulations can be directly molded or cast into quite complex product shapes without substantial requirements for working the formed product.
These products have ordinarily been quite heavy, imposing considerable costs for shipping, handling, installation, and the like. It has long been an objective of the art to find effective techniques to reduce the weight of such products.
In addition, the high filler loadings employed in order to achieve marble-like properties and appearance have resulted in low physical properties, including tensile strength, impact and crack resistance, yield strength, and the like. While these properties may be improvements over marble itself, they tend to be poor compared to other filled polymer products generally.
It has become common to employ hollow ceramic or glass spheres as a part of the filler component in such systems to reduce weight. Such hollow spheres are friable, particularly during the vigorous mixing required in formulating such compositions, and while some weight reductions are achieved by virtue of such inclusions, the effect is often limited by the lack of strength of such additions. In addition, the physical properties achieved are not greatly improved by hollow glass or ceramic spheres.
The weight reductions that are achieved by such techniques are limited by the size and by the friability of the hollow glass or ceramic spheres. In most usual circumstances, such materials have a diameter which averages about 100 micrometers, and a distribution of sizes from as little as 10 to as much as 350 micrometers.
In U.S. Pat. Nos. 4,839,394 and 4,849,456, (Champion) the use of hollow glass or ceramic spheres is improved upon by combining these materials with hollow thermoplastic microspheres. The glass or ceramic spheres are blended with the thermoplastic microspheres, and mixed to induce an electrostatic attraction such that the microspheres are attracted to the surface of the larger glass or ceramic spheres. In subsequent mixing operations, the microspheres furnish a cushioning effect, reducing the fragmentation of the more fragile glass or ceramic material. In the molded or cast products, the microspheres are said to improve the impact resistance and other related physical properties.
The thermoplastic microspheres employed in the foregoing patents are hollow spheres of a thermoplastic polymer which enclose a physical blowing agent, such as a volatile hydrocarbon. The most commonly available and widely used of such materials are polyvinylidene chloride homopolymer or copolymer based materials.
In Champion, the breakage of the glass or ceramic spheres is reduced, but not eliminated. In addition, the amount that can be employed continues to be limited, and the weight savings attained is consequently less than might otherwise be desirable. There is some gain in physical properties of the products, in impact and crack resistance, but the products remain quite fragile. In addition, the coating of the hollow ceramic spheres with the microspheres limits the binding of the polymer binder to the surface of the ceramic material, leaving these materials only partially bonded into the final product. This results in undesirable levels of voids, air entrainment, potential problems with the polymer curing system, and the creation of sites for fracture, crack initiation, and crack propagation through the cast product.